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Notes From the Road

The Skinny on Run-flat Tires
By Aaron Robinson

The Black Art of Black Rubber
Tuning tires comes down to forging a series of compromises between conflicting forces to get the ride and performance balance ideal for your new model. Here are some examples: stiffer sidewalls create better performance because the tire deforms less in corners and under acceleration and braking. That creates the kind of crisp control responses you expect in a sports car. But, stiffer tires also transmit more bump energy into the chassis, which makes for a harsher ride. Larger tires have a bigger “contact patch,” the vital link between the car and Mother Earth. The bigger the contact patch, the more grip you have and the better the car turns, accelerates, and brakes. But, it also generates more noise, cuts fuel economy, and has worse sloppy-weather traction. Softer rubber compounds in the tread provide better grip, but wear out more quickly.

And so on…

Car engineers are accustomed to juggling these tradeoffs to get the ride and handling they want. With run-flats, it’s a whole new world. The stiffer sidewalls of a run-flat, for example, mean smaller contact patches and more bump-energy transmission. Indeed, the first run-flat tires were cited for their low grip and harsh ride. As demand for run-flats has increased, the car industry is becoming smarter about how to adapt the technology.

For starters, the tire makers are devising better rubber formulas that reduce the ride penalty caused by the reinforced sidewalls. Driving a 2005 Ford Mustang GMT equipped with Pirelli’s Eufori@ tires through the villages of southern France turned a lot of heads but didn’t bobble ours over rough pavement. The automakers have also started tuning their suspensions specifically for run-flats. One tweak is relaxing the “durometer,” or stiffness of the suspension bushings, to help cushion impacts that would better absorbed by a conventional tire. 

What Does it All Mean?
If your car didn’t come with run-flats originally, proceed with caution. Run-flats work best with suspensions that are designed for them from the outset. You may find the ride too stiff for your liking. If the new car you are shopping for has run-flats, forget the negative press you have read in the past. Run-flat technology has rolled a long way since the early days, and it’s going to continue. Part of Pirelli’s display included a prototype wheel rim cast with a hollow chamber that is pumped with air. If the tire develops a leak, a warning signal lights on the dash while a valve allows air to pass from the chamber to the tire, maintaining tire pressure until you can safely pull over. Another system in development constantly monitors air pressures, calling the driver on his or her cell phone if the pressures drop below normal.

Wait. Tires that call you on your cell phone? It’s not just dumb rubber any more.

Aaron Robinson has spent the past 15 years reporting on cars and the car industry, first for a national car dealer magazine, then for the weekly trade newspaper "Automotive News," now as Technical Editor of the world's largest consumer car monthly, "Car and Driver." When not hunched over a word processor, he's usually found in his garage trying to save worthless old cars from a well-deserved trip to the junkyard.

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